Modifying tree nutrition for optimal storage quality
Lime and fertiliser recommendations for apples are intended to ensure that availability and uptake of mineral nutrients do not limit growth and cropping. Soil and leaf analyses are used to determine general fertiliser requirements and to identify mineral deficiencies.
However, it cannot be assumed that a nutrient supply that is satisfactory for tree growth and cropping necessarily produces fruit with a satisfactory storage potential and eating quality. In many cases the availability of specific minerals needs to be restricted or supplemented in order to improve storage and eating quality.
Growers are advised to analyse soils, leaves and fruits on a regular basis in order to understand the full effect of their use of lime and fertilisers in specific orchards. These different types of analyses are necessary, as often there is a lack of association between fertiliser application and nutrient uptake or a poor correlation between nutrient levels in leaves and fruits.
There are many factors other than fertiliser application that affect the levels of nutrients in the leaves and fruits. Soil type, water availability, rootstock, use of green waste mulches, pruning and cropping are some of the more important factors that influence uptake and partitioning of mineral nutrients within the tree.
The effect of composted green waste on tree nutrition
AHDB Horticulture funded Project TF 177 to assess the long-term effects of applied composted green waste mulch on the cropping of Braeburn and Cox apple orchards. Trees growing in a bare herbicide strip in a Braeburn and Cox orchard in Kent were compared with those mulched with composted green waste. The effects of the mulch were assessed on fruit size, weight, number per tree, maturity, mineral analysis, storage potential and soil moisture.
The mulch had a significant impact on fruit size particularly in Cox where, in every year of the project, fruit from trees that had been mulched were up to 6mm larger compared with fruit from untreated trees. For Braeburn, the effect was more variable.
In both Cox and Braeburn, the mulch led to an increase in fruit numbers resulting in significantly increased yields and equivalent returns per hectare. The mulch also led to more shoot growth in both varieties with young trees filling their space in the orchard earlier than untreated trees.
The compost treatment led to an increase in fruit nitrogen levels and a reduced level of calcium. This could have an adverse effect on the long-term storage potential of the fruit. Growers using such mulches must ensure that they analyse fruit mineral content every year before harvest and also analyse leaf and soil samples routinely and apply soil and foliar feeds accordingly.
Fruit on compost-treated trees matured around two weeks earlier than fruit on untreated trees, measured by an earlier drop in percentage starch and fruit firmness. There were also changes to fruit colour. Fruit from mulched trees did not show as much red colour and the background was a darker green although this was more a result of the fruit nitrogen content rather than any effect on maturity. Fruit from compost-treated orchards needs to be harvested separately and maturity tests conducted on farms to determine the optimum harvest date for individual orchards.
Nutritional guidelines that combine adequate cropping with good storage potential:
- In Cox orchards apply sufficient nitrogen fertiliser to achieve a maximum nitrogen concentration of 2.6% (dry weight) in leaves and 70 mg 100g-1 (fresh weight) in fruit at harvest.
- In Bramley orchards apply sufficient nitrogen fertiliser to achieve a minimum nitrogen concentration of 2.6% (dry weight) in leaves (M9 only) and a maximum nitrogen concentration 60 mg 100g-1 (fresh weight) in fruit at harvest. Bramley on MM106 should have a maximum leaf N concentration of 2.6%.
- Economise on the use of nitrogen fertiliser by minimising grass/weed competition for water and nitrogen.
- Supplement phosphorus nutrition in orchards where there is a history of flesh breakdown in stored fruit and where low phosphorus is implicated in the problem. Proprietary products are available for this purpose. These include ‘Seniphos’ (Yara Phosyn Ltd) that has proved particularly effective in raising phosphorus levels and reducing low temperature breakdown in Cox and Bramley apples.
- In Cox orchards, achieve a minimum phosphorus concentration of 0.24% (dry weight) in leaves and raise leaf phosphorus concentrations in Bramley to the higher end of the suggested range (0.18-0.23%).
- Apply phosphorus sprays in mid-June to mid-July to reduce susceptibility of fruits to breakdown in store. This is particularly important for Bramley where a third of orchards achieve fruit phosphorus levels below the recommended threshold of 9 mg 100g-1 (fresh weight).
Potassium supply in orchards should be sufficient for adequate growth and cropping but major adverse effects occur during storage in fruit with excessive potassium.
- Potassium deficiency may be corrected by applying 3 sprays of potassium sulphate at 14-day intervals from petal fall. In Gala and Jonagold it may be appropriate to apply potassium nitrate if nitrogen levels are low.
- The application of calcium sprays should be regarded as routine on Cox, Bramley, Egremont Russet and Spartan. The effectiveness of the programme should be judged against mineral composition standards for good storage quality.
- Calcium chloride is the preferred salt for most cultivars for the control of bitter pit and related disorders. A minimum programme would apply 72 kg hectare-1 of 78% flaked grade calcium chloride per season.
- In Egremont Russet, 110 kg hectare-1 of calcium nitrate prills (79% calcium nitrate) per season is preferred in order to avoid leaf scorch associated with the use of the chloride form.
- In Cox, the nitrate form of calcium is preferred as fruit firmness is likely to be retained more effectively than with the use of the chloride form. Care is required in its application to avoid any lenticel injury to the fruit.
- To derive maximum effect from calcium sprays, apply the full amount of material per season regardless of water volume and apply from June to as near harvest as is practicable. Minimise leaf scorch from calcium chloride by spraying in cool temperatures (<21oC) or by reduced spray concentration. Where spray rates are reduced increase the frequency of spraying. It may not be necessary to be as cautious about spraying in high temperatures when applying calcium chloride in water volumes of 100 litres per hectare or less.
- Proprietary products such as ‘Wuxal type 2′ and ‘Calcium Metalosate’ have given improved results over standard flake calcium chloride when applied at equivalent rates of calcium per hectare. Other proprietary calcium products may provide similar improvements in efficacy but evidence should be provided to this effect before these are used in preference to calcium chloride (or calcium nitrate) and particularly where lower rates of calcium per hectare are advised.
- Foliar application of ‘PreTect’ is claimed by the manufacturers (Plant Health Care plc) to increase fruit calcium concentrations. Data provided showed calcium increases of 66% and 47% in Golden Delicious and Fuji apples respectively.
- Supplement magnesium nutrition where leaf analysis indicates sub-optimal levels or where there are visible symptoms of deficiency. Low magnesium in fruits may induce flesh breakdown in stored fruit whilst excessive levels increase susceptibility to bitter pit and related disorders.
- Two to five applications of magnesium sulphate (20 kg 1000 l-1 ha-1) applied at 14 day intervals provide a more rapid control of magnesium deficiency than soil applied forms of magnesium. It is important to offset any increased bitter pit potential from the magnesium sprays by subsequently applying a full calcium spray programme.
- Other proprietary products are available to rectify deficiencies of magnesium. Growers should consider the most appropriate material and method of application for their particular orchards in consultation with their advisers and spray representatives.
- Although boron deficiency can cause serious corking problems in apples, this is rare in the UK. There are adverse effects on storage quality that result from an over supply of boron. Analysis of soils, leaves and fruits should be carried out when boron deficiency is suspected.
- Boron deficiency can be rectified by soil application of materials such as borax (20 kg ha-1) or ‘Solubor’ (10 kg ha-1) in the spring or by 3 sprays of ‘Solubor’ (2 kg 1000 l-1 ha-1), with an appropriate wetter, starting at petal fall and repeated at 2‑3 week intervals.
- Other proprietary products are available to rectify deficiencies of boron. Growers should consider the most appropriate material and method of application for their particular orchards in consultation with their advisers and spray representatives.